1. Field of the Invention
The present invention relates to a carburetor to be used for an internal combustion engine equipped with a supercharger. In particular, a carburetor is designed to be connected at one end of a suction passage thereof to the discharge port of a supercharger and at the other end thereof to the intake manifold of an internal combustion engine. The carburetor has a main fuel nozzle opening into a venturi section formed in the suction passage to mix the air supplied from the supercharger with fuel according to the degree of opening of a throttle valve disposed downstream of the venturi section.
2. Description of the Prior Art
In a carburetor of an internal combustion engine equipped with no supercharger, an air current is formed in the suction passage thereof by a negative pressure produced in the intake manifold of the internal combustion engine. The negative pressure prevails in that part of the suction passage which is downstream of the throttle valve while the internal combustion engine is running. Accordingly, even if a throttle shaft fixed to the throttle valve is pivotally supported in and led outside of the body of the carburetor through a through hole formed in the body of the carburetor in order to connect the throttle shaft to the accelerator pedal through a linkage, there is no possibility of fuel leaking outside of the body of the carburetor through a clearance between the outer circumference of the throttle shaft and the inside wall of the through hole.
When a supercharger is provided upstream of the suction passage of the body of the carburetor in order to supercharge the internal combustion engine, the internal pressure of the intake manifold of the internal combustion engine sometimes becomes higher than the atmospheric pressure due to the operation of the supercharger. Consequently, there is a hazardous possibility of fuel, which is flowing within the suction passage of the carburetor and the intake manifold, being ejected and leaking together with air through the clearance in the throttle shaft supporting portion. In order to avoid such a hazardous possibility, it may be devised to form an air vent in the body of the carburetor so as to open at one end thereof into the suction passage at a position upstream of the venturi section and to open at the other end thereof into the through hole formed in the body of the carburetor for pivotally supporting the throttle shaft so that the supercharging pressure of the supercharger is applied to the clearance between the through hole and throttle shaft. However, even in such a construction, it is impossible to stop the ejected leak of fuel through the clearance thoroughly, because the pressure within the suction passage in the vicinity of the throttle valve fluctuates due to the pulsative variation of the internal pressure of the intake manifold of the internal combustion engine resulting from the cyclic operation of the internal combustion engine. In this constitution, the ejected leak of fuel can perfectly be stopped by the provision of a venturi section of a reduced diameter, however, reduction in the diameter of the venturi section impedes the flow of the suction air current through the suction passage of the carburetor causing the reduction of the output power of the internal combustion engine.